Chemistry, Third edition

REDOX COUPLES

2.asalt bridge(a glass tube that contains a saturated solution of an ionic salt, such

as KNO 3 ) which completes the electrical circuit.

If we measure the voltage of the cell (the cell potential) using a high-resistance

voltmeter, we will not be using up any electrical current produced by the cell, and

very little chemical reaction takes place. Under these conditions, the measured volt-

age does not fall during the measurement.

Which electrode is connected to the positive terminal of the voltmeter? By trial

and error, it is found that only if the Ag(aq)/Ag(s) electrode is connected to the

positive terminal is the cell potential positive. (A voltage of 0.80 V is actually

obtained; if the connections were reversed, the voltage reading would be 0.80 V.)

Because of this, the Ag(aq)/Ag(s) electrode is said to be the cell cathode() and

the hydrogen electrode the cell anode().

Electrons flow from the anode to the cathode. This means that the hydrogen elec-

trode is losing electrons (oxidation); this is only possible if the H(aq)/H 2 (g) redox

couple is undergoing the reaction:

H 2 (g)2H(aq)2e

The electrons then pass through the circuit and move to the cathode where they cause

the Ag(aq)/Ag(s) redox couple to be reduced within the Ag(aq)/Ag(s) electrode:

Ag(aq)eAg(s)

The overall cell reaction is:

H 2 (g)2Ag(aq)2H(aq)2Ag(s)

(The same reaction occurs outside a cell, if hydrogen gas is bubbled into a solution

containing silver ions. However, in that case heat – not electricity – is made.)

Thecell diagramfor any electrochemical cell follows the pattern:

reactantproduct reactant product

()anode  ()cathode

(in which oxidation takes place) (in which reduction takes place)

wheresymbolizes the salt bridge. In our case:

PtH 2 (g) H(aq) Ag(aq) Ag(s)

goes to goes to

which shows that H 2 (g) is oxidized to H(aq) (the product at the anode), while

Ag(aq) is reduced to Ag(s) (the product at the cathode).

The standard potential of the cell, E°=0.80 V, is related to the standard elec-

trode potentials for each redox couple by the following equation:

E°=E°RE°L

which applies to all cells, and in which ER°is the standard electrode potential of

the right-hand electrode (cathode) and E°Lthe standard electrode potential of the

left-hand electrode (anode) as they appear in the cell diagram. Here, E°R=

E°(Ag(aq)/Ag(s)) and EL°=E°(H(aq),H 2 (g)) = 0 (by definition). Thus:

0.80 V = E°(Ag/Ag(s)) 0

andE°(Ag/Ag(s)) = 0.80 V.

2. Finding E°(Zn^2 /Zn(s))

Suppose we wanted to measure E°(Zn^2 (aq)/Zn(s)). Our first step is to set up the

electrochemical cell, consisting of the SHE and standard (Zn^2 (aq)/Zn(s)) electrode

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